Besides differences in the schema, there are several other differences between the earlier Office XML schema formats and Office Open XML.

Whereas the data in Office Open XML documents is stored in multiple parts and compressed in a ZIP file conforming to the Open Packaging Convention, Microsoft Office XML formats are stored as plain single monolithic XML files (making them quite large, compared to OOXML and the Microsoft Office legacy binary formats). Also, embedded items like pictures are stored as binary encoded blocks within the XML. In case of Office Open XML, the header, footer, comments of a document etc. are all stored separately.

Poor backward compatibility with the version of Word/Excel prior to the one in which they were introduced. For example, Word 2002 cannot open Word 2003 XML files unless a third-party converter add-in is installed.[3] Microsoft has released a Word 2003 XML Viewer which allows WordProcessingML files saved by Word 2003 to be viewed as HTML from within Internet Explorer.[4] For Office Open XML, Microsoft provides converters for Office 2003, Office XP and Office 2000.

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